Add Behat feature to test forum tool as a platform admin

tests/features/forum.feature

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+Feature: Forum tool
+  In order to use the Forum tool
+  The teachers should be able to create forum categories, forums, forum threads
+
+  Background:
+    Given I am a platform administrator
+    And I am on course "TEMP" homepage
+
+  Scenario: Create a forum category
+    Given I am on "/main/forum/index.php?action=add&content=forumcategory"
+    When I fill in the following:
+      | forum_category_title   | Forum Category Test                       |
+      | forum_category_comment | This is the first forum category for test |
+    And I press "SubmitForumCategory"
+    Then I should see "The forum category has been added"
+
+  Scenario: Create a forum
+    Given I am on "/main/forum/index.php?action=add&content=forum"
+    When I fill in the following:
+      | forum_title   | Forum Test                       |
+      | forum_comment | This is the first forum for test |
+    And I press "SubmitForum"
+    Then I should see "The forum has been added"
+
+  Scenario: Create a forum thread
+    Given I am on "/main/forum/newthread.php?forum=5"
+    When I fill in the following:
+      | post_title | Thread One                                     |
+      | post_text  | This is a the first thread in a forum for test |
+    And I press "SubmitPost"
+    Then I should see "The new thread has been added"